The present invention relates to a method and apparatus for determining a direction of arrival of a signal. The invention is applicable but not limited to finding the direction of arrival of an electromagnetic signal such as a radio signal.
In many applications it is of interest to determine the direction of arrival of a signal. An example is in a radio communication system wherein the determination of a direction of arrival of a received signal from one communication unit can be used to maximise the power transmitted in that direction by a second communication unit. This allows optimisation of the signal level of the radio communication signal transmitted from the second communication unit when received by the first communication unit.
An example of a radio communication system wherein this method can be applied is a cellular mobile communication system such as the Global System for Mobile Communication. In a cellular mobile communication system each of the mobile stations communicate with typically a fixed base station. Communication from the mobile station to the base station is known as uplink and communication from the base station to the mobile station is known as downlink. The total coverage area of the system is divided into a number of separate cells each covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area. As a mobile station moves from the coverage area of one cell to the coverage area of another cell, the communication link will change from being between the mobile station and the base station of the first cell to being between the mobile station the base station of the second cell. This is known as a handover.
Specifically, some cells may lie completely within the coverage of other larger cells. These are known as hierarchical cells and an example is the so called microcells which are used to provide a high traffic capacity in high traffic area. Typically the microcells are small and a large number of cells can be implemented in a limited area. A mobile station moving into the microcell will be handed over from the overlaying cell, known as the macrocell. This frees up resource from the macrocell and hierarchical cell thus provide the possibility of a large coverage area combined with high traffic capacity.
All base stations are interconnected by a network. This network comprises communication lines, switches, interfaces to other communication networks and various controllers required for operating the network. A call from a mobile station is routed through this network to the destination specific for the call. If the call is between two mobile stations of the same communication system, it will be routed through the network to the base station currently serving the other mobile station currently. A connection is thus established between the two serving cells through the network. Alternatively, if the call is between a mobile station and a telephone connected to the Public Switched Telephone Network (PSTN) the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.
A cellular mobile communication system is allocated a frequency spectrum for the radio communication between the mobile stations and the base stations. This spectrum must be shared between all mobile station simultaneously using the system. In GSM and similar systems, this is achieved by dividing the spectrum into a number of frequency channels. In GSM each of the frequency channels are furthermore divided into eight distinct time slots. By allocating a time slot to each active mobile station, eight mobile stations can thus be served by each frequency channel. This approach is known as Time Division Multiple Access (TDMA). Each cell is allocated a number of frequency channels. As the number of frequency channels are limited, the same frequency channels are typically allocated to more than one cell. This is known as frequency re-use and the tighter the frequency reuse, i.e. the closer together the same frequency channel can be used, the higher is the achievable traffic capacity of the system.
The quality of the radio communication between the mobile station and the base station is determined by the signal to noise level of the signals. Other base stations and mobile stations generate interference which increases the noise level and thus reduced the quality. In order to attain an acceptable quality level, the interference must be kept sufficiently low. The interference can be generated from transmissions on the same frequency channel which is known as so-called interference. Alternatively, it can be generated from transmissions on adjacent channels as it is not possible prevent unwanted emissions outside the allocated frequency channel. This interference is known as adjacent channel interference. As the interference level reduces with increasing distance to the interferer, the interference level will be increased for tighter frequency re-use. Today""s cellular communication system incorporates other measures to minimise interference levels such as power control which reduces the power transmitted to the lowest level that will provide adequate link quality.
Another important method of reducing interference is by using directional antennas whereby power is mainly transmitted in the direction for optimal signal quality at the desired receiver. When receiving, a directional antenna will attenuate the interference received from the other directions thereby allowing the transmitter to transmit at lower power.
Directional antennas are often physically large and expensive and therefore impractical at the mobile station, and most directional antennas are deployed at the base stations. The most suitable directional antennas are antenna arrays consisting of a plurality of antenna elements. By individually adjusting a relative phase correction for each element the antenna array will have a directional beam pattern. The operation of antenna arrays are described in xe2x80x98Introduction to adaptive arraysxe2x80x99 by Monzingo and Miller, 1980, Wiley Publishing.
A known method for reducing interference in a cellular communication system is to determine a direction of arrival of an uplink signal received by an antenna array, and transmitting the downlink signal in the direction determined. However, the benefit of the method is very dependent on the accuracy and reliability of the direction of arrival estimate obtained. Known methods for estimating a direction of arrival have a high degree of unreliability and a system for improving the reliability and provide a way of determining the reliability of the estimate is desirable.
The current invention seeks to provide a method and apparatus for improving the reliability and accuracy of an estimate of a direction of arrival and to determine the reliability of the obtained estimate.
According to an aspect of the present invention, there is provided a method of determining a direction of arrival of a received signal including the steps of obtaining a first estimate of a direction of arrival of the received signal from a first sensor, obtaining at least second estimate of a direction of arrival of the received signal from at least a second sensor and characterised by comprising the step of, determining a direction of arrival estimate of the received signal as a function of both said first and at least second estimate.
Preferably, the first and at least second estimate represent the direction of arrival estimate when the difference between the first and at least second estimate is below a threshold.
According to a feature of the present invention a downlink signal is transmitted in the direction of the direction of arrival estimate in a cellular communication system.
According to another aspect of the present invention, there is provided an apparatus for determining a direction of arrival of a received signal including a first estimator generating a first estimate of a direction of the received signal from a first sensor, at least a second estimator generating a second estimate of a direction of the received radio signal from a second sensor and characterised by including, a controller determining a direction of arrival estimate of the received radio signal as a function of both said first and at least second estimate.